Magnetic recording medium

ABSTRACT

A magnetic recording medium is described wherein a layer comprising mainly a polyurethane resin, vinylidene chloride copolymer, or a cellulose derivative is formed on the surface of a vapor-deposited ferromagnetic metal layer on a substrate.

BACKGROUND OF THE INVENTION

This invention relates to a magnetic recording medium, and particularly to a protective layer for a magnetic recording medium of the vapor deposition type.

A magnetic recording medium of the vapor deposition type is manufactured by physical and chemical vapor deposition, such as vacuum deposition, sputtering, or ion plating, wherein a ferromagnetic metal is heated in vacuum or a gaseous atmosphere until it vaporizes and deposits physically on a support. Such magnetic recording medium constantly contacts and slides on the transport system and magnetic head in a recording/reproduction apparatus, and it must have high wear resistance, abrasion resistance and lubricity, but none of the conventional products known to date meets these requirements perfectly.

Several methods are known for providing a protective layer for improving the wear resistance of the magnetic recording medium, for example, the technique of electroplating a rhodium film, nickel film, nickel-phosphorus film or nickel-tin film, the technique of oxidizing the surface of a thin cobalt-containing ferromagnetic metal film by exposing it to a suitable temperature and moisture (Japanese Patent Publication No. 20025/67 and U.S. Pat. No. 3,353,166), the technique of contacting a thin magnetic alloy film with nitric acid, treating the same with heat to form an oxide film on the surface, and impregnating the film with a lubricant (British Pat. No. 1,265,175), and the technique of depositing the vapor of chromium on the surface of a thin ferromagnetic metal film in a suitable degree of vacuum to thereby form a layer composed of a mixture of chromium and chromium oxide (Japanese Patent Publication No. 4393/70).

Methods are also known to form a protective layer by vapor deposition, sputtering, or ion plating SiO (U.S. Pat. Nos. 3,109,746, 3,353,166, and Japanese Patent Application (OPI) No. 80102/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), SiO₂ (Japanese Patent Application (OPI) Nos. 93404/75 and 112307/76), noble metals (Japanese Patent Application (OPI) Nos. 93604/75 and 58304/76), and metal oxides, carbides, nitrides, sulfides and fluorides (Japanese Patent Application (OPI) Nos. 104602/75, 123408/75, 146303/75, 56202/76, 21901/78, 21902/78 and 30304/78), but none of the resulting protective layers are completely satisfactory.

SUMMARY OF THE INVENTION

Therefore, the primary object of this invention is to provide a magnetic recording medium of the vapor deposition type having high wear resistance, abrasion resistance, and a good running property (referring to tape transport in a recording/reproduction apparatus).

As a result of studies to achieve this object, it has now been found that the desired magnetic recording medium can be achieved by forming a layer comprising mainly a polyurethane resin, vinylidene chloride copolymer, or a cellulose derivative on the surface of a vapor-deposited ferromagnetic metal layer on a substrate.

The reason why a protective layer comprising mainly a polyurethane resin or cellulose derivative is particularly effective has not been elucidated, but one possible reason is that it is hard and has high glass transition point (Tg). A layer comprising mainly a vinylidene chloride copolymer has a high ability to shield the ferromagnetic metal layer from gas such as oxygen, so it is expected to be more stable against oxidation than layers of other polymers of the same thickness.

DETAILED DESCRIPTION OF THE INVENTION

The polyurethane resin, the vinylidene chloride copolymer and the cellulose derivative used in this invention have an average molecular weight in the range of from about 5,000 to about 500,000, and preferably from 10,000 to 200,000.

Typical examples of the polyurethane are one prepared from a polyisocyanate and a polyester made from a polybasic organic acid and polyol, and one prepared from polyether glycol or polyoxyalkylene glycol and polyisocyanate. Preferred polyurethanes are those prepared from polyesters such as adipic acid/butanediol, adipic acid/hexanediol, adipic acid/diethylene glycol, sebacic acid/glycol, and sebacic acid/butanediol, and polyisocyanates such as toluene diisocyanate, hexamethylene diisocyanate, and diphenylmethane diisocyanate. The polyurethane described in Japanese Patent Application No. 26880/79 is also applicable. Such polyurethanes are commercially available from Nippon Polyurethane Industry Co., Ltd. under the trademarks Paraprene 22S, 26S, 22SM, 25SM, and Nipporan 2301, from Bayer AG., West Germany, under the trademark Desmolak 2100, and Morton Chemical, U.S.A., under the trademarks CA 250HV, CA275 and CA300.

The vinylidene chloride copolymer used in this invention comprises from 98 to 60 wt%, and preferably from 95 to 70 wt%, vinylidene chloride. Examples of the monomer copolymerizable with vinylidene chloride include acrylonitrile, vinyl chloride, an acrylic ester wherein the alkyl group has 1 to 6 carbon atoms, and a methacrylic ester wherein the alkyl group has 1 to 6 carbon atoms.

The cellulose derivatives used in the present invention are those having an OH group content of from 7.5% to 40%. Examples of the cellulose derivative used in this invention include nitrocellulose, cellulose acetobutyrate, cellulose acetate propionate, cellulose acetate and ethyl cellulose, which are known in the art, for example, in U.S. Pat. No. 3,630,771.

These polymers are dissolved in a solvent such as an ester, ketone, aromatic hydrocarbon, dimethylformamide (DMF), or tetrahydrofuran (THF), in case of the polyurethane; with tetrahydrofuran, methyl ethyl ketone (MEK), ethyl acetate, or cyclohexanone, in case of the vinylidene chloride copolymer; or in an ester, ketone, or alcohol, in case of the celluose derivative. The coating solution thus-formed is coated on the surface of the ferromagnetic metal layer on the substrate.

If desired, the polyurethane, cellulose derivative, and vinylidene chloride copolymer may be combined to provide a coating having improved surface properties. In this case, the major component of the protective layer may be combined with from 0.2 to 30 wt%, based on the total weight of the protective layer, of an additional component. If the content of the additional component (e.g., the cellulose derivative or vinylidene chloride copolymer when the major component is polyurethane) is too great, the resulting protective layer has low wear and abrasion resistance and a poor running property perhaps due to the decrease in the inherent properties of the major component (e.g., polyurethane). Thus, the protective layer preferably contains at least 70 wt% of a polyurethane resin, a vinylidene chloride copolymer or a cellulose derivative.

If desired, the coating solution may contain from 0.2 to 30 wt% of a diluting agent such as a hydrocarbon (e.g., toluene, xylene or mineral spirit), a chloride (e.g., carbon tetrachloride or ethylene dichloride), or an alcohol (e.g., ethanol or isopropanol) or a suitable plasticizer to provide intimate contact with the metal layer such as dioctyl phthalate (DOP), dibutyl phthalate (DBP), triphenyl phosphate (TPP), polymeric plasticizer or thermoplastic polyurethane resin.

The object of this invention is achieved more effectively when a lubricant is incorporated into the protective layer or is coated on the protective layer. Examples of the lubricant include aliphatic acids, aliphatic acid esters, paraffin waxes, metal soaps, higher alcohols, amines of aliphatic acids, amide derivatives, fluorine compounds and silicon compounds.

To achieve the desired protecting effect and to prevent output drop that is caused by spacing loss due to the gap between the magnetic recording surface and magnetic head, the protective layer preferably has a thickness of from about 0.002 to 0.3 μm, and preferably from 0.005 to 0.2 μm.

The thin ferromagnetic metal film used in this invention is prepared by vapor deposition of a metal such as Fe, Co or Ni, or a ferromagnetic alloy such as Fe-Co, Fe-Ni, Co-Ni, Fe-Co-Ni, Fe-Rh, Fe-Cu, Co-Cu, Co-Au, Co-Y, Co-La, Co-Pr, Co-Gd, Co-Sm, Co-Pt, Ni-Cu, Mn-Bi, Mn-Sb, Mn-Al, Fe-Cr, Co-Cr, Ni-Cr, Fe-Co-Cr and Fe-Co-Ni-Cr. The magnetic film must be thick enough to provide an output necessary for a magnetic recording medium and must be thin enough to achieve the desired high-density recording, so generally the film has a thickness between 0.02 and 5.0 μm, and preferably between 0.05 and 2 μm. The vapor deposition of such a metal or alloy can be effected by the procedure as disclosed in U.S. Pat. No. 3,342,632.

Examples of the substrate or base include polymeric bases such as polyester, polyethylene terephthalate, polyimide, polyamide, polyamideimide, polyvinyl chloride, triacetyl cellulose, polycarbonate and polyethylene terephthalate, and metal substrates such as Al, Al alloy, Ti, Ti alloy and stainless steel. Another example of the substrate is magnetic tape composed of a support coated with a layer of finely divided ferromagnetic material and a binder.

Further, when the protective layer is hard in nature, a polymer layer having a relatively low Young's modulus, i.e., about 10 to about 500 kg/mm², can be provided as a primer between the ferromagnetic layer and the protective layer in order to absorb external force whereby adhesion of the protective layer to the ferromagnetic layer can be improved. Examples of such polymers are polyesters, vinylidene chloride/acrylonitrile copolymers and the like.

This invention is now described in greater detail by reference to the following examples, which are provided here for illustrative purposes only, and are not intended to limit the scope of the invention. In the examples, all parts are by weight.

EXAMPLE 1

A thin layer of Co-Ni was vapor-deposited in a continuous manner at 6×10⁻⁵ Torr on a polyethylene terephthalate film (15μ thick) in a thickness of 1,500 Å. That thin layer was further coated with a protective layer from a coating solution of the following composition to provide a layer having a dry thickness of 0.1μ by gravure coating.

    ______________________________________                                         Composition of Coating Solution                                                ______________________________________                                         Polyurethane resin      100    parts                                           (Desmolak 2100 of Bayer AG.)                                                   Methyl isobutyl ketone (MIBK)/                                                                         1,000  parts                                           methyl ethyl ketone (MEK)(1/1)                                                 ______________________________________                                    

Before application of the coating solution, the thin layer of Co-Ni was coated with a primer to form a primer layer having a dry thickness of 0.1 μm from a solution in methylene dichloride of an amorphous polyester resin prepared from terephthalic acid.

The magnetic recording medium obtained was slit into tape sample No. 1 1/2 inch wide, which was run on a small-scale video tape recorder [Video Home System (VHS) VTR deck] to check its abrasion resistance, wear resistance, and running property. Abrasion resistance of the magnetic recording medium was evaluated by counting the number of abrasions developed after 20 repeated runs. Wear resistance was evaluated by checking the wear of the tape surface after 20 repeated runs. Running property was checked by determining the dynamic friction coefficient from the tensions measured at both the incoming and outgoing ends of the cylinder. The results are shown in Table 1.

                                      TABLE 1                                      __________________________________________________________________________                             Wear                                                                               Running Property                                                           Resist-                                                                            (dynamic) Friction                                           Abrasion Resistance                                                                          ance                                                                               Coefficient                                                  After         After                                                                              (25° C., 60% RH)                            Sample                                                                             Virgin                                                                               5   After                                                                               After                                                                               20th                                                                               1st                                                                               5th                                                                               10th                                                                              20th                                      No. Tape  Runs                                                                               10 Runs                                                                             20 Runs                                                                             Run Run                                                                               Run                                                                               Run                                                                               Run                                       __________________________________________________________________________     1   0     0   one  one  good                                                                               0.24                                                                              0.22                                                                              0.21                                                                              0.21                                                    shallow                                                                             shallow                                                                   abrasion                                                                            abrasion                                                    __________________________________________________________________________

Table 1 shows the test sample has high resistance to abrasion and wear, and achieves substantially stable running (i.e., there is little change in the dynamic friction coefficient).

EXAMPLES 2 TO 5

Tape sample Nos. 2 to 5 were prepared analogous to the procedure of Example 1, using a composition of the vapor-deposited film, base, and protective layer, as described in Table 2 below. In these Examples, the protective layer was coated in the same thickness as that of Example 1 (0.1μ dry thickness).

                                      TABLE 2                                      __________________________________________________________________________     Sample                                                                             Vapor-Deposited Film and Base                                                                    Composition of Protective Layer                          __________________________________________________________________________     2   Co film (thickness = 1,000 Å)                                                                Polyurethane resin (Desmolak 2100)                                                                100                                                                               parts                                  Polyimide base (thickness = 12 μm)                                                            Butyl acetate      0.4                                                                               part                                                     MEK/Toluene (1/1)  1,000                                                                             parts                              3   Co--V film (V = 10 wt %,                                                                         Polyurethane resin 100                                                                               parts                                  thickness = 1,200 Å)                                                                         (Paraprene 22S of Nippon Polyurethane                                          Industry Co., Ltd.)                                          Polyethylene terephthalate base                                                                  MEK/Cyclohexanone (3/7)                                                                           1,000                                                                             parts                                  (thickness = 16 μm)                                                     4   Co--Fe Film (Fe = 15 wt %,                                                                       Polyurethane resin 90 parts                                  thickness = 1,500 Å)                                                                         (Paraprene 22S of Nippon Polyurethane                                          Industry Co., Ltd.)                                          Polyethylene terephthalate base                                                                  Nitrocellullse (RS 1/2)                                                                           10 parts                                  (thickness = 20 μ m)                                                                          MEK/Cyclohexanone (3/7)                                                                           1,000                                                                             parts                              5   Co--Cr film (Cr = 2 wt %,                                                                        Polyurethane resin 100                                                                               parts                                  thickness = 1,200 Å)                                                                         (Paraprene 22S of Nippon Polyurethane                                          Industry Co., Ltd.)                                          Polyethylene terephthalate base                                                                  Oleic acid         0.4                                                                               part                                   (thickness = 20 μm)                                                                           MEK/Cyclohexanone (3/7)                                                                           1,000                                                                             parts                              __________________________________________________________________________

COMPARATIVE EXAMPLES A-C

Comparative Samples A, B, and C were prepared analogous to the procedure of Example 1, using a composition of the vapor-deposited film, base, and protective coating as described in Table 3 below.

                  TABLE 3                                                          ______________________________________                                         Sample  Vapor-Deposited                                                                             Composition of                                            No.     Film and Base                                                                               Protective Layer                                          ______________________________________                                         A       The same as in                                                                              No protective layer                                               Sample No. 1.                                                          B       The same as in                                                                              1 wt % solution of butyl                                          Sample No. 2.                                                                               stearate (as liquid lubri-                                                     cant) in MEK/MIBK (1/1) was                                                    applied and dried.                                                             Coating weight of butyl                                                        stearate = 20 mg/m.sup.2                                  C       The same as in                                                                              Aliphatic acid modified                                           Sample No. 3.                                                                               silicone oil (as liquid                                                        lubricant) was applied as                                                      in Sample B to give a coating                                                  weight of 15 mg/m.sup.2.                                  ______________________________________                                    

The wear resistance, abrasion resistance, and running property of Sample Nos. 2 to 5 and Samples A, B, and C were evaluated by the same methods as in Example 1. The results are shown in Table 4 below.

                                      TABLE 4                                      __________________________________________________________________________                                     Running                                                                        Property (dynamic                                                              friction coefficient)                          Abrasion Resistance             (25° C. 65% RH)                         Sample                                                                             Virgin                                                                             After                                                                               After                                                                               After                                                                               Wear Resistance                                                                         1st                                                                               5th                                                                               10th                                                                              20th                                  No. Tape                                                                               5 Runs                                                                              10 Runs                                                                             20 Runs                                                                             After 20 Runs                                                                           Run                                                                               Run                                                                               Run                                                                               Run                                   __________________________________________________________________________     2   0   0    0    1 shallow                                                                           Good     0.22                                                                              0.21                                                                              0.20                                                                              0.19                                                    abrasion                                                     3   0   0    1 shallow                                                                           1 shallow                                                                           "        0.20                                                                              0.21                                                                              0.20                                                                              0.19                                               abrasion                                                                            abrasion                                                     4   0   0    0    1 shallow                                                                           "        0.22                                                                              0.21                                                                              0.20                                                                              0.21                                                    abrasion                                                     5   0   0    0    0    "        0.22                                                                              0.19                                                                              0.18                                                                              0.18                                  A   0   10 deep                                                                             23 deep                                                                             49 deep                                                                             Worn too greatly                                                                        0.21                                                                              0.22                                                                              0.29                                                                              0.45                                          abrasions                                                                           abrasions                                                                           abrasions                                                                           to be evaluated                                         B   0   8 deep                                                                              19 deep                                                                             36 deep                                                                             Poor     0.23                                                                              0.22                                                                              0.21                                                                              0.22                                          abrasions                                                                           abrasions                                                                           abrasions                                                    C   0   7 deep                                                                              15 deep                                                                             41 deep                                                                             "        0.20                                                                              0.19                                                                              0.21                                                                              0.20                                          abrasions                                                                           abrasions                                                                           abrasions                                                    __________________________________________________________________________

EXAMPLE 6

Sample No. 6 was prepared by repeating the procedure of Example 1, except that a coating solution of the following composition was used to provide the protective layer.

    ______________________________________                                         Coating Solution                                                               ______________________________________                                         Vinylidene chloride/acrylonitrile                                                                      100    parts                                           (weight ratio = 87:13) copolymer                                               MIBK/MEK (2/8)          1,000  parts                                           ______________________________________                                    

The abrasion resistance and running property of the sample were evaluated by the same method as in Example 1. The results are shown in Table 5 below.

                                      TABLE 5                                      __________________________________________________________________________                           Running Property                                         Abrasion Resistance   (dynamic friction coefficient)                           Sample                                                                             Virgin                                                                             After                                                                              After                                                                               After                                                                               (25° C. 60% RH)                                   No. Tape                                                                               5 Runs                                                                             10 Runs                                                                             20 Runs                                                                             1st Run                                                                             5th Run                                                                             10th Run                                                                            20th Run                                  __________________________________________________________________________     6   0   0   1 shallow                                                                           2 shallow                                                                           0.28 0.27 0.25 0.25                                                  abrasion                                                                            abrasions                                                     __________________________________________________________________________

Table 5 shows that the sample had good abrasion resistance and achieved stable running (i.e., little change in the dynamic friction coefficient).

EXAMPLES 7 TO 10

Sample Nos. 7 to 10 were prepared analogous to the procedure of Example 6, using a composition of the vapor-deposited film, base, and protective layer, as described in Table 6 below.

                                      TABLE 6                                      __________________________________________________________________________     Sample                                                                             Vapor-deposited Film and Base                                                                    Composition of Protective Layer                          __________________________________________________________________________     7   Co film (thickness = 1,000 Å)                                                                Vinylidene chloride/vinyl chloride                                                                95 parts                                                    (78/22) copolymer                                            Polyimide base (thickness = 12 μm)                                                            Nitrocellulose (RS 1/2)                                                                           5  parts                                                    MEK/THF (1/1)      1,000                                                                             parts                              8   Co--V film (V = 10 wt %,                                                                         Vinylidene chloride/ethylmethacrylate                                                             100                                                                               parts                                  thickness = 1,200 Å)                                                                         (95/5 wt %) copolymer                                        Polyethylene terephthalate base                                                                  Oleic acid         0.5                                                                               part                                   (thickness = 16 μm)                                                                           MEK/MIBK (7/3)     1,000                                                                             parts                              9   Co--Fe film (Fe = 15 wt %,                                                                       Vinylidene chloride/acrylonitrile                                                                 90 parts                                  thickness = 1,500 Å)                                                                         (80/20 wt %) copolymer                                       Polyethylene terephthalate base                                                                  Vinyl chloride acetate resin                                                                      10 parts                                  (thickness = 20 μm)                                                                           (MPR-TA of Nisshin Chemical Industry                                           Co., Ltd.)                                                                     THF/Toluene (1/1)  1,000                                                                             parts                              10  C0--Cr film (Cr = 2 wt %,                                                                        Vinylidene chloride/acrylonitrile                                                                 100                                                                               parts                                  thickness = 1,200 Å)                                                                         (80/20 wt %) copolymer                                       Polyethylene terephthalate base                                                                  THF/Toluene (1/1)  1,000                                                                             parts                                  (thickness =  20 μm)                                                    __________________________________________________________________________

COMPARATIVE EXAMPLES D-F

Comparative Samples D, E, and F were prepared analogous to the procedure of Example 6, using a composition of the vapor-deposited film, base, and protective coating as described in Table 7 below.

                  TABLE 7                                                          ______________________________________                                         Sample  Vapor-Deposited                                                                             Composition of                                            No.     Film and Base                                                                               Protective Layer                                          ______________________________________                                         D       The same as in                                                                              No protective layer                                               Example 6.                                                             E       The same as in                                                                              0.2 wt % solution of butyl                                        Example 7.   stearate (as liquid lubricant)                                                 in MEK/MIBK was applied and                                                    dried.                                                                         Coating weight of butyl                                                        stearate = 20 mg/m.sup.2                                  F       The same as in                                                                              Aliphatic acid modified                                           Example 8.   silicone oil (as liquid                                                        lubricant) was applied as in                                                   Sample B to give a coating                                                     weight of 15 mg/m.sup.2.                                  ______________________________________                                    

The abrasion resistance and running property of Sample Nos. 7 to 10 and Comparative Samples D to F were evaluated by the same methods as in Example 1. The results are shown in Table 8 below.

                                      TABLE 8                                      __________________________________________________________________________     Abrasion Resistance    Running Property                                        Sample                                                                             Virgin                                                                             After                                                                               After                                                                               After                                                                               (dynamic friction coefficient)                          No. Tape                                                                               5 Runs                                                                              10 Runs                                                                             20 Runs                                                                             1st Run                                                                             5th Run                                                                             10th Run                                                                            20th Run                                 __________________________________________________________________________     7   0   0    1 shallow                                                                           1 shallow                                                                           0.27 0.25 0.24 0.24                                                  abrasion                                                                            abrasion                                                     8   0   0    1 shallow                                                                           2 shallow                                                                           0.28 0.26 0.25 0.25                                                  abrasion                                                                            abrasions                                                    9   0   0    0    1 shallow                                                                           0.29 0.27 0.25 0.25                                                       abrasion                                                     10  0   0    0    1 shallow                                                                           0.28 0.25 0.25 0.25                                                       abrasion                                                     D   0   8 deep                                                                              20 deep                                                                             50 deep                                                                             0.29 0.30 0.41 0.48                                             abrasions                                                                           abrasions                                                                           abrasions                                                    E   0   7 deep                                                                              15 deep                                                                             40 deep                                                                             0.27 0.27 0.28 0.27                                             abrasions                                                                           abrasions                                                                           abrasions                                                    F   0   6 deep                                                                              15 deep                                                                             40 deep                                                                             0.26 0.25 0.25 0.26                                             abrasions                                                                           abrasions                                                                           abrasions                                                    __________________________________________________________________________

The results of Table 8 show that the magnetic tape samples of this invention have high abrasion resistance and good running property. More abrasions developed on Samples D, E, and F as the number of runs increased, but very few abrasions developed on Sample Nos. 7 to 10. The dynamic friction coefficient of Sample D, having no protective lubricant, increased as the number of tape runs increased, but no great increase occurred in the dynamic friction coefficient of Samples E and F. However, both Samples E and F wore greatly due to their poor abrasion resistance.

EXAMPLE 11

Sample No. 11 was prepared by repeating the procedure of Example 1, except that a coating solution of the following composition was used to provide a protective layer.

    ______________________________________                                         Coating Solution                                                               ______________________________________                                         Nitrocellulose (RS 1/2)                                                                               80     parts                                            DOP (dioctyl phthalate)                                                                               20     parts                                            MIBK/MEK (1/1)         1,000  parts                                            ______________________________________                                    

The abrasion resistance, wear resistance and running property of the samples were evaluated by the same methods as in Example 1. The results are shown in Table 9.

                                      TABLE 9                                      __________________________________________________________________________                           Wear Running Property                                    Abrasion Resistance   Resistance                                                                          (dynamic friction coefficient)                      Sample                                                                             Virgin                                                                             After                                                                              After                                                                               After                                                                               After                                                                               (25° C. 65% RH)                              No. Tape                                                                               5 Runs                                                                             10 Runs                                                                             20 Runs                                                                             20 Runs                                                                             1st Run                                                                             5th Run                                                                             10th Run                                                                            20th Run                             __________________________________________________________________________     11  0   0   1 shallow                                                                           1 shallow                                                                           Good 0.22 0.22 0.21 0.20                                             abrasion                                                                            abrasion                                                      __________________________________________________________________________

Table 9 shows that the magnetic tape samples of this invention have high resistance to abrasion and wear and achieve stable running (i.e., little change in the dynamic friction coefficient).

EXAMPLES 12 TO 15

Sample Nos. 12 to 15 were prepared analogous to the procedure of Example 11, using a composition of the vapor-deposited film, base, and protective layer as described in Table 10 below.

                                      TABLE 10                                     __________________________________________________________________________     Sample                                                                             Vapor-Deposited Film and Base                                                                    Composition of Protective Layer                          __________________________________________________________________________     12  Co film (thickness = 1,000 Å)                                                                Nitrocellulose (RS 20)                                                                           75 parts                                   Polyimide base (thickness = 12 μm)                                                            Butylene diadipate polyurethane                                                                  25 parts                                                     (Mn 15,000)                                                                    MEK/Toluene (1/1) 1,000                                                                             parts                               13  Co--V film (V = 10 wt %,                                                                         Cellulose acetate butyrate (1/2 sec)                                                             90 parts                                   thickness = 1,2000 Å)                                                      Polyethylene terephthalate base                                                                  Vinyl acetate resin                                                                              10 parts                                   (thickness = 16 μm)                                                                           (degree of polymerization = 450)                                               Cyclohexanone/MEK (1/1)                                                                          1,000                                                                             parts                               14  Co--Fe film (Fe = 15 wt %,                                                                       Nitrocellulose (H 1/2)                                                                           82 parts                                   thickness = 1,500 Å)                                                                         DBP               18 parts                                   Polyethylene terephthalate base                                                                  Dimethyl polysiloxane                                                                            0.5                                                                               part                                    (thickness = 20 μm)                                                                           Butyl acetate/ethyl acetate (1/1)                                                                1,000                                                                             parts                               15  Co--Cr film (Cr = 2 wt %,                                                                        Cellulose acetate propionate (1/2 sec)                                                           85 parts                                   thickness = 1,200 Å)                                                                         Triphenyl phosphate                                                                              15 parts                                   Polyethylene terephthalate base                                                                  Oleic acid        0.5                                                                               part                                    (thickness =  20 μm)                                                                          MEK/MIBK (1/1)    1,000                                                                             parts                               __________________________________________________________________________

COMPARATIVE EXAMPLES G-I

Comparative Samples G, H, and I were prepared analogous to the procedure of Example 11, using a composition of the vapor-deposited film, base, and protective layer, see Table 11 below.

                  TABLE 11                                                         ______________________________________                                         Sample  Vapor-Deposited                                                                             Composition of                                            No.     Film and Base                                                                               Protective Layer                                          ______________________________________                                         G       The same as in                                                                              No protective layer                                               Sample No. 11.                                                         H       The same as in                                                                              1 wt % solution of butyl                                          Sample No. 12.                                                                              stearate (as liquid lubricant)                                                 in MEK/MIBK (1/1) was applied                                                  and dried.                                                                     Coating weight of butyl                                                        stearate = 20 mg/m.sup.2.                                 I       The same as in                                                                              Aliphatic acid modified                                           Sample No. 13.                                                                              silicone oil (as liquid lubri-                                                 cant) was applied as in                                                        Sample H to give a coating of                                                  15 mg/m.sup.2.                                            ______________________________________                                    

The abrasion resistance, wear resistance, and running property of Sample Nos. 12 to 15 and Comparative Examples G to I were evaluated by the same methods as in Example 1. The results are shown in Table 12.

                                      TABLE 12                                     __________________________________________________________________________                                     Running                                                                        Property (dynamic                                                              friction coefficient)                          Abrasion Resistance             (25° C., 65% RH)                        Sample                                                                             Virgin                                                                             After                                                                               After                                                                               After                                                                               Wear Resistance                                                                         1st                                                                               5th                                                                               10th                                                                              20th                                  No. Tape                                                                               5 Runs                                                                              10 Runs                                                                             20 Runs                                                                             After 20 Runs                                                                           Run                                                                               Run                                                                               Run                                                                               Run                                   __________________________________________________________________________     12  0   0    0    1 shallow                                                                           Good     0.20                                                                              0.20                                                                              0.19                                                                              0.19                                                    abrasion                                                     13  0   0    1 shallow                                                                           1 shallow                                                                           "        0.19                                                                              0.18                                                                              0.19                                                                              0.19                                               abrasion                                                                            abrasion                                                     14  0   0    0    1 shallow                                                                           "        0.21                                                                              0.20                                                                              0.19                                                                              0.19                                                    abrasion                                                     15  0   0    0    0    "        0.22                                                                              0.19                                                                              0.18                                                                              0.18                                  G   0   10 deep                                                                             23 deep                                                                             49 deep                                                                             Worn too greatly                                                                        0.20                                                                              0.23                                                                              0.28                                                                              0.41                                          abrasions                                                                           abrasions                                                                           abrasions                                                                           to be evaluated                                         H   0   8 deep                                                                              19 deep                                                                             36 deep                                                                             "        0.20                                                                              0.21                                                                              0.21                                                                              0.21                                          abrasions                                                                           abrasions                                                                           abrasions                                                    I   0   7 deep                                                                              15 deep                                                                             41 deep                                                                             "        0.18                                                                              0.19                                                                              0.21                                                                              0.20                                          abrasions                                                                           abrasions                                                                           abrasions                                                    __________________________________________________________________________

The results of Table 12 show that the magnetic tape samples according to the present invention have high resistance to abrasion and wear and excellent running property.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. 

What is claimed is:
 1. A magnetic recording medium comprising(a) a support, (b) a vapor-deposited ferromagnetic metal layer on said support(a), and (c) a protective layer comprising mainly one of (i) a polyurethane resin, (ii) a vinylidene chloride copolymer or (iii) a cellulose derivative on the surface of said vapor-deposited ferromagnetic metal layer (b), said protective layer having a thickness of from about 0.002 to 0.3 μm.
 2. A magnetic recording medium as in claim 1, wherein the protective layer (c) has a thickness of from about 0.005 to 0.2 μm.
 3. A magnetic recording medium as in claim 1, or 2, wherein the protective layer (c) comprises mainly a polyurethane, and from 0.2 to 30 wt% of a vinylidene chloride copolymer or cellulose derivative, based on the total weight of the protective layer, as an additional component thereof.
 4. A magnetic recording medium as in claim 1, or 2 wherein the protective layer (c) comprises mainly a polyurethane resin having an average molecular weight in the range of from 5,000 to 500,000.
 5. A magnetic recording medium as in claim 4, wherein the average molecular weight is from 10,000 to 200,000.
 6. A magnetic recording medium as in claim 1, or 3, wherein the protective layer (c) comprises mainly a vinylidene chloride copolymer comprising from 98 to 60 wt% vinylidene chloride.
 7. A magnetic recording medium as in claim 6, wherein the vinylidene chloride copolymer comprises from 95 to 70 wt% vinylidene chloride.
 8. A magnetic recording medium as in claim 1, or 2, wherein the protective layer (c) contains a lubricant.
 9. A magnetic recording medium as in claim 1, or 2, comprising a primer layer (d) between the ferromagnetic metal layer (b) and the protective layer (c). 